In a typical roofing installation, an underlayment is applied between the roof deck and the exterior roof covering (e.g. asphalt shingles). The underlayment separates the shingles from the roof deck and provides a secondary water shedding barrier and weather protection for the roof. Traditionally, the underlayment is an asphalt impregnated felt or paper product, though now, many roof installers are using synthetic polymer underlayment sheets.
Whether asphalt impregnated paper or synthetic polymer underlayment, the underlayment sheets are typically sold in a roll to the roof installer. The standard width of the roll of underlayment is 48 inches. When installed, rows of underlayment are laid parallel to the eaves. The installer unrolls the underlayment on a roof, cuts it to the proper length, such as the width of the roof, and secures the underlayment to the roof utilizing staples or nails. Each subsequent course of underlayment is lapped over the underlaying course and secured to the roof deck and so on until the roof deck is covered. On roofs with intersecting sloped roof planes, each sloped plane is covered in a similar fashion.